Reverse voltage circuit for thyristors

ABSTRACT

The invention relates to a reverse voltage circuit for thyristors which has particular application to an inverter circuit. The basic circuit includes a thyristor, a first diode, and a transformer having the primary winding thereof in series relation to the diode. The first diode and the transformer primary winding are in parallel relation to the thyristor with the diode being antiparallel relative to the thyristor. The transformer secondary winding and a second diode are in series and a D.C. voltage source biases the second diode in a counter direction relative to the secondary winding. With this circuit higher reverse voltages are obtained having relatively lengthy durations to effect switching off of a thyristor in a satisfactory manner.

United States Patent 5 Q Petersen et al.

[541 REVERSE VOLTAGE CIRCUIT FOR THYRISTORS [72] Inventors: Tom Kas'trup Petersen; Neils Juul l-lenriksen, both of Nordborg, f; 1 Denmark7" [7 3] Assignee': Danfoss AIS, Nordborg, Denmark 22 P111311;DR.28,1970

[21] Appl.No.: 101,599

[30] Foreign Application Priority Data Dec. 29', 1969 Denmark... 6686i52 vs. c1 ..321/45 11, 307/252 M, 321/45 c,

511 1m. 01. ..H02m 7/52 [58] Field of Search ..32l/2, 45 R, 45 C, 45 ER;

323/22 SC, 23, 24, 25, 38, 307/252 M, 252 T j [56] References CitedUNITED STATES PATENTS 3,629,673 12/1971 Thorborg ..321/45 0 3,641,3642/1972 Rippel "307/252 M 3,308,371

3/1967 Studtmann,Jr. ..32l/45 ER 1151 3,701,939 Z1451 Oct. 31, 1972 vStudtmannnfm, ..321/45 0 3,405,346 10/1968 Krauthamer... ..321/4scFOREIGN PATENTS on APPLICATIONS 1,174,086 12/1969 Great Britain .321/45c Primary ExaminerGerald Goldberg Attorney-Wayne B. Easton f 1571ABSTRACT The invention relates to a reverse voltage circuit forthyristors which has particular application to an inverter circuit.Thebasic circuit includes a thyristor, a first diode, and a transformerhaving the primary winding thereof in series relation to the diode. Thefirst diode and the transformer primary winding are in parallel relationto the thyristor with the-diode being antiparallel relative to thethyristor. The transformer secondary winding and a second diode are inseries and a DC. voltage source biases the second diode in a counterdirection relative to the secondary winding. With this circuit higherreverse voltages are obtained having relatively lengthy durations toeffect switching off of a thyristor in a satisfactory manner.

3 Claims, 3 Drawing Figures mimibncrsi1912" SHEET 2 OF 2 REVERSE VOLTAGECIRCUIT FOR THYRISTORS The invention concerns a reverse voltage circuitfor thyristors and of the kind in which a diode is connected in parallelwith the thyristor but with current flowing through in the oppositedirection. Such thyristor circuits are normally connected to acommutation circuit controlling the thyristor, and have the disadvantagethat they require a thyristor of high quality and operating on very lowreverse voltages, i.e. the voltage at which the thyristor is effectivelyswitched off.

A further problem was that of obtaining voltage impulses of sufficientlylengthy duration so that switching off could be achieved effectively.With these heavy currents in particular, the stray inductance can causereduction in voltage in conductors, including those running, to thereverse voltage diode, so that the thyristor is not switched off in asatisfactory manner and the period during which voltage occurs is toobrief.

It is also possible'in other circuits to obtain much higher voltageswhich are in fact higher than is necessary for switchingoff thethyristor, so that the cost of the circuit is consequently increased.

The object of the invention is to provide a circuit in which a highervoltage can be obtained and in which the magnitude of the voltage itselfcan be determined to some extent, so that even thyristors havingrelatively poor characteristics, particularly the magnitude of thereverse voltage, do not constitute a very critical factor.

According to the invention, this object is achieved by connecting inseries with the diode the primary winding of a transformer having atleast a primary and a secondary winding, and the secondary winding ofwhich is connected through a second diode to a DC. voltage source whichbiases the second diode, and in that the voltage induced in thesecondary winding of the transformer runs counter to the bias of thesecond diode. Thus, the switch-off current which flows through the diodeconnected in parallel with the thyristor is caused to induce a voltageinthe transformer which voltage, because of the self-induction of thetransformer, instantaneously rises to a high level and cancels out thebias in the second diode, and the primary voltage thus occurring in thetransformer can beset to a quite high level, so that the magnitude ofthe voltage is adjusted to suit the thyristor. It is also expedient ifthe DC. voltage source is constituted by a constant voltage supply/lnthis way, it becomes possible to keep the voltage substantially constantover a longer span of time, and in fact until current flows in the diodeconnected in parallel with the thyristor. This constant voltage can beobtained in a particularly simple manner if the voltage source has acondenserfilter output section.

Further simplification of such equipment is achieved by the DC voltagesource being constituted by a common supply voltage, and by a condenserbeing connected in parallel overthe output section of the DC. voltagesource. This renders it possible for the common voltage supply to beused for the entire circuit in order to provide the voltage, and at thesame time the energy or the greater part of the energy, used in thecourseof commutation, is recouped.

In the case of thyristor inverters, which comprise thyristorphase-branches consisting of thyristors connected in series and havingno-load diodes connected in parallel with the thyristors, use can bemade of this circuit since it is characterized in that the primarywindings of the transformer are directly connected in series and theircommon junction constitutes a phase a voltage tap-off point, in that theno-load diode-cathode of the first thyristor is connected to the anodeof the first thyristor, and the anode of the no-load diode is connectedto the common point of the second primary winding and of the anode ofthe second thyristor, and in that the anode of the second no-load diodeis connected to the cathode of the second thyristor, and the cathode ofthe second no-load diode is connected to the common point of the firstprimary winding and of the cathode of the first thyristor. In thisarrangement, the primary windings of the transformers can replace theknown center-point choke, while all the advantages of the circuit areretained. Circuits of this kind are known. US. In inverters of this kindit is advantageous, in the case of those types in which the thyristorphasebranches are jointly reversed, to connect in series the secondarywindings of the transformer in the phasebranches, and for the seconddiode and the DC. voltage source to be common to all the thyristorbranches. In this way, a saving is achieved in that, for example in thecase of three-phase inverters, only one instead of three diodes isrequired, while at the same time, the energy is channelled back to thevoltage supply for the equipment. In such equipment it is alsoadvantageous to connect the secondary windings in parallel in eachbranch, since this leads to a better transmission ratio in thetransformers as well as to reduction of the total voltage requiredforthe DC. voltage source that biases the diode.

- The invention will now be described in more detail by reference to thedrawing, in which:

FIG. 1 illustrates the principle of the circuit,

FIG. 2 shows the current curve, and

FIG. 3 is a diagram relating to the inverter in accordance with theinvention.

The thyristor E, is inserted between a voltage supply V, and a consumerunit, not illustrated, an operating current 1,, flowing towards thisunit. The diode D, is connected in parallel with the thyristor but inthe opposite direction, and a primary winding of a transformer Tr, isconnected in series with this diode. The secondary of the transformer isconnected in parallel, through a second diode D with a condenser C,which capacitor is charged to a level corresponding to the value of thesupply voltage V,.

When the thyristor is to be switched off, a current l, can be passedfrom a commutation circuit K through a switch S. This current is dividedinto the operating current I, and the reverse current 1,, which passesthrough the thyristor E,. This current also supplies a current l whichpasses through the diode D, and the primary winding of the transformerTr,. The current I, will now provide a quite large voltage by way of theprimary of the transformer because of the self-induction of the latter.This voltage now induces, in the winding directions indicated by dots, avoltage in the secondary of the transformer opposed to the voltage V,,which biases the diode D The voltage now rises almost instantaneously toa value which is greater than V,, and a voltage, which is substantiallyconstant and dependent upon the transmission ratio of the transformer,is generated on the primary side of the transformer as long as thecurrent I is flowing. In the case here illustrated in which the supplyvoltage is used, a suitable transmission ratio is one in the order ofmagnitude of l or more. The reverse voltage over the thyristor E, is

now the sum of the voltage-drop at the diode, amounting to l 1.5 Volts,and of the voltage over the transformer, amounting to 30 40 Volts,depending upon the transmission ratio. In contrast to other circuits, acertain controllable voltage is here obtained as a counter-voltage overthe thyristor, which counter-voltage helpsitoclose the thyristor in aneffective manner, even in the case of thyristors where the reversevoltage is very critical, since the values of their parameters canfluctuate somewhat. A similar circuit is used in the inverter shown inFIG. 3 where a voltage supply S,, provides the inverter with voltagethrough the choke D, and by way of the conductors 5 and 6. A commutationunit K is arranged in parallel over these conductors and in series withan interruptor A which is used for jointly reversing all thephase-branches. The three phasebranches here shown are identical, eachcontaining two controlled thyristors E,, E E and E E and E connected inseries. Between the two series-connected thyristors is inserted theprimary winding of two transformers, and the common middle pointsbetween the transformers Tr, and Tr,, Tr, and Tr,, Tr and Tr,,constitute the phase-voltage tappings U, V, and W. In branch 1 theprimary windings P, and P are thus connected in series with the windingdirections indicated by the dots. The same applies in the case of theother branches of the windings P P4, P and P In parallel over the seriesconnections'E, P, P E,, P P E P P,,, the diodes D,, D and D areconnected in the opposite direction in relation to the thyristors E Eand E The diodes D D and D are connected in parallel over the seriesconnections P, P E,, P P, E,, and P P E The diodes D, D are no-loaddiodes for the inverter. The secondaries in the individualphase-branches are connected in parallel with the winding directionindicated by the dots, e.'g. in the phase-branches U at 8,. Theparallel-connected secondary winding S, and S S and S S and S areconnected in series with each other in pairs through the conductors 1,2, 3 and 4. The conductors 3 and 4 are again connected through the diodeD to the condenser C, which is charged to the voltage V, with thepolarity indicated by the arrow. The diode D and the resistor Rconstitute a damping member. The inverter is jointly reversed throughthe commutation circuit K and the interruptor A, and if the interruptorA is closed, a current I flows through the conductor 6. This currentdivides into part-currents I,, I and l 3 in the various phasebranches.In the phase-branch U for example the current I, flows through the diodeD the primary windings P, and P and the diode D, and back to thecommutation circuit. As this happens, a voltage V and V having thepolarity indicated is induced in the primary windings, and this voltagerises rapidly because of the self-induction of the transformer when notloaded, and an induced voltage having a polarity indicated by the arrow;occurs in the secondary of the transformer. The sum of these voltages isopposed to the voltage V, in the winding directions indicated, and whenthis voltage exceeds the voltage V,, a current flows through the diode Dand as long as the current I, is flowing a substantially constantvoltage occurs in the phase-branch U by way of the primary windings P, PThis voltage can drop somewhat because of the possible strayselfinduction of the transformer. The voltages occurring in eachphase-branch during switching off bias the thyristors E,, E E E E andE,,, so that the thyristors are effectivelyswitched off. The resultantvoltage impulse is of considerable duration and in fact is equal toapproximately percent of the switch-off period. This impulse is thusmore suitable, the characteristics of the thyristors used being quiteuncritical as regards their reverse voltage. At the same time theself-inductionof the supply leads and the effect of other self-inductionis considerably suppressed, since the voltages V, and V,, are nowdetermined by the transmission ratio of the transformer and by thevoltage V,, and can therefore be of a magnitude necessary for providingreasonable safety. In the case of earlier connections it was necessaryto grade these thyristors, whereas they are now quite uncritical becauseof the high voltage of between 30 40 volts now available, while at thesame time, the volt- I age or voltage impulse is of greater duration.The voltage across the condenser C can either be obtained from aseparate voltage source, preferably a constant voltage source, or fromthe voltage supply for the inverter. In this way, the commutation energynow used is returned directly'to the current supply, so that it is notlost, and this greatly improves the economics of the equipment. This isachieved by making the broken-line connection from the anode of diode 7to the anode 'of diode 6 and by connecting the conductor 4 to thepositive terminal of the voltage supply. The principle upon which theinverter here illustrated is based is described in our US. Pat. No.3,559,034'granted Jan. 26, 1971 and titled Inverted Converter HavingCommon Quenching Means For All Controllable Rectifiers.

What is claimed is:;'

1. A reverse voltage circuit for a thyristor comprising: a thyristor; atransformer having a primary winding and a secondary winding; a firstconductor path connected in parallel with said thyristor, said firstpath in cluding said primary winding in series with a first diode poledin the reverse direction relative to said thyristor; a second conductorpath connected in parallel with said thyristor, said second pathincluding commutator means and switch means which is closed under thecontrol of said commutator means when a reverse voltage is to be appliedacross said thyristor, closure of said switch means causing a current toflow in said first path in the forward conducting direction of saidfirst diode; a second diode and a capacitor connected in series withsaid secondary winding; and charging means for charging said capacitorin a sense to apply a reversevoltage across said second diode; closureof said switch means causing a current to flow insaid primary winding toinduce a voltage in said secondary winding sufficient to overcome thereverse voltage across said second diode whereby current flows in saidsecondary winding and a reverse voltage substantially in excess of theforward voltage drop of the first diode is applied across saidthyristor.

2. A reverse voltage circuit according to claim 1 wherein said chargingmeans is a constant voltage D.C. supply.

3. An inverter comprising at least one thyristor phase branch with firstand second thyristors in series; first and second transformers eachhaving primary and control of said commutator means when a reversevoltage is to be applied across said thyristors; a third diode and acapacitor connected in series with said secondary windings; and chargingmeans for charging said capacitor in a sense to apply a reverse voltageacross said third diode; closure of said switch means causing a currentto flow in said primary windings to induce a voltage in said secondarywindings sufficient to overcome the reverse voltage across the thirddiode whereby current flows in said secondary windings and a reversevoltage substantially in excess of the forward voltage drops of thefirst and second diodes is applied across said thyristors.

1. A reverse voltage circuit for a thyristor comprising: a thyristor; atransformer having a primary winding and a secondary winding; a firstconductor path connected in parallel with said thyristor, said firstpath including said primary winding in series with a first diode poledin the reverse direction relative to said thyristor; a second conductorpath connected in parallel with said thyristor, said second pathincluding commutator means and switch means which is closed under thecontrol of said commutator means when a reverse voltage is to be appliedacross said thyristor, closure of said switch means causing a current toflow in said first path in the forward conducting direction of saidfirst diode; a second diode and a capacitor connected in series withsaid secondary winding; and charging means for charging said capacitorin a sense to apply a reverse voltage across said second diode; closureof said switch means causing a current to flow in said primary windingto induce a voltage in said secondary winding sufficient to overcome thereverse voltage across said second diode whereby current flows In saidsecondary winding and a reverse voltage substantially in excess of theforward voltage drop of the first diode is applied across saidthyristor.
 2. A reverse voltage circuit according to claim 1 whereinsaid charging means is a constant voltage D.C. supply.
 3. An invertercomprising at least one thyristor phase branch with first and secondthyristors in series; first and second transformers each having primaryand secondary windings, said primary windings being in series with eachother and with said thyristors, said primary windings being between saidthyristors, and the common junction of said primary windings forming aphase voltage tapping; a first diode in parallel with said primarywindings and said first thyristor; a second diode in parallel with saidprimary windings and said second thyristor, said diodes being inanti-parallel relation respectively to said thyristors; a path connectedin parallel with said branch, said path including commutator means andswitch means which is closed under the control of said commutator meanswhen a reverse voltage is to be applied across said thyristors; a thirddiode and a capacitor connected in series with said secondary windings;and charging means for charging said capacitor in a sense to apply areverse voltage across said third diode; closure of said switch meanscausing a current to flow in said primary windings to induce a voltagein said secondary windings sufficient to overcome the reverse voltageacross the third diode whereby current flows in said secondary windingsand a reverse voltage substantially in excess of the forward voltagedrops of the first and second diodes is applied across said thyristors.